1. Field of the Invention
The present invention relates to a process for producing an elongated superconductor such as a superconducting wire. More particularly, it relates to a novel process for producing an elongated superconductor composed of a sintered compound oxide or a metal oxide complex having an improved mechanical strength as well as possessing a high critical temperature of superconductivity.
2. Description of the Related Art
Under the superconducting condition, the electric resistance become zero and the perfect diamagnetism is observed and electric current of a very high current density can be delivered without any loss of power.
Realization of superconducting coils which permit to generate a very high magnetic field is expected to accelerate development in the field of fusion power generation as well as in the field of MHD power generation or motor-generators. The development of superconducting coils is demanded also in the other industrial fields such as electric power reservation system; transportation systems such as magnetic levitation trains or magnetically propelling ships; medical appliances such as a high-energy beam radiation unit; or scientific instruments such as NMR.
In addition to the abovementioned electric power applications, the superconducting materials can be used in the field of electronics, for example, as a device using the Josephson effect in which quantum efficiency is observed macroscopically when an electric current is passed through a weak junction arranged between two superconductors. Tunnel junction type Josephson device which is a typical application of the Josephson effect is expected to be a high-speed and low-power consuming switching device owing to smaller energy gap of the superconducting material. It is also expected to utilize the Josephson device as a high sensitive sensors or detectors for sensing very weak magnetic field, microwave, radiant ray or the like since variation of electromagnetic wave or magnetic field is reflected in variation of Josephson effect and can be observed as a quantum phenomenon precisely. Development of the superconducting devices is also demanded in the field of high-speed computers in which the power consumption per unit area is reaching to the upper limit of the cooling capacity with increment of the integration density in order to reduce energy consumption. However, the critical temperature of superconductivity could not exceed 23.2 K. of Nb.sub.3 Ge which was the highest Tc for the past ten years.
The possibility of an existence of new types of superconducting materials having much higher Tc was revealed by Bednorz and Muller, who discovered a new oxide type superconductor in 1986 [Z. Phys. B64 (1986) 189].
It had been known that certain ceramic materials of compound oxides exhibit the property of superconductivity. For example, U.S. Pat. No. 3,932,315 discloses Ba-Pb-Bi type compound oxide which shows superconductivity and Japanese patent laid-open No. 60-173,885 discloses that Ba-Bi type compound oxides also show superconductivity. These superconductors, however, possess rather lower transition temperatures of about 10 K. and hence usage of liquidized helium (boiling point of 4.2 K.) as cryogen is indispensable to realize superconductivity.
The new type compound oxide superconductor discovered by Bednorz and Muller is represented by [La, Sr].sub.2 CuO.sub.4 which is called the K.sub.2 NiF.sub.4 -type oxide having a crystal structure which is similar to known perovskite type oxides. The K.sub.2 NiF.sub.4 -type compound oxides show such higher Tc as 30 K., which are extremely higher than known superconducting materials.
It was also reported that C. W. Chu et al discovered, in the United States of America, another superconducting material so called YBCO type represented by YBa.sub.2 Cu.sub.3 O.sub.7-x having the critical temperature of about 90 K. in February 1987. Still other type new superconducting materials which were reported recently are a compound oxide of Bi-Sr-Ca-Cu-O system and Tl-Ba-Ca-Cu-O system which exhibit such high Tc as more than 100 K. and which are chemically much stable than the abovementioned YBCO type compound oxide or the like. And hence, the possibility of an existence of high-temperature superconductors have burst onto the scene.
However, these new type superconducting materials are ceramic materials of compound oxides or metal oxide complexes which do not possess such a high plasticity as well-known metal type superconducting materials such as Ni-Ti alloy. In fact, they are produced by a sintering technique in which a compact of powder material is sintered in a furnace. However, the sintered articles are fragile and easily brake even under a very weak mechanical stress, so that they can not or are difficult to be shaped or deformed into a desired shape such as a wire by the conventional plastic deformation techniques. In order to realize a reliable and practical superconducting structure, it is indispensable that the structure possesses enough strength and tenacity which is sufficient to endure bending force during usage.
Taking the abovementioned situation into consideration, the present inventors have proposed a process for manufacturing a ceramic wire by the steps comprising filling a metal pipe with a material powder of ceramic, performing plastic deformation of the metal pipe filled with the ceramic metal powder by wire-drawing or forging technique to reduce the cross section of the metal pipe so that the metal pipe is shaped into a desired configuration and the density of the material powder in the pipe is increased, and then subjecting the deformed metal pipe to heat-treatment to sinter the ceramic material powder in the metal pipe in an European patent application No. 88 400 257.6. This process is very effective and advantageous for producing superconducting wires because the outer metal pipe possesses satisfactory mechanical strength against outer stress and functions as a by-pass circuit when the superconductivity break down.
However, we found such a fact that the superconducting property of the product obtained by this process is inferior to that of a sintered block which is produced by sintering the material powder outside the metal pipe in air or in an oxygen atmosphere. This may be caused by insufficient oxygen deficiency in the crystal of the sintered product obtained. Namely, it is difficult to control the oxygen contents in the crystal, if the material powder is sintered in a closed atmosphere in the metal pipe, in other words, without exposing the material powder to oxygen atmosphere during the sintering stage.
In fact, it is known that the oxygen contents in the crystal is a critical factor for realizing desired properties of the superconductor and hence the precise control of the oxygen contents in a surrounding atmosphere during the sintering stage is indispensable. For this purpose, the material powder is heated in an oxygen atmosphere.
A typical process for producing a superconducting sintered block comprises the following steps.
At first, a powder mixture of compounds of constituent metal elements of which the superconducting sintered block is composed, for example a mixture of metal oxides is shaped into a compact. Then, the compact is heated under a partial oxygen pressure of about 1 atom to sinter the compact. The resulting sintered mass is cooled down or annealed slowly to 300.degree. to 400.degree. C. under the same oxygen partial pressure for from several hours to a dozen or so hours. Finally, the sintered mass is cooled to a room temperature. The annealing stage is believed to be an indispensable stage for realizing and stabilizing the superconducting property having a desired high critical temperature because the oxygen contents in the crystal obtained is influenced by the annealing condition.
However, as is described above, in the case that the material powder is sintered inside the metal pipe, it is difficult or impossible to exposed the material powder to the surrounding oxygen atmosphere during the annealing stage.
In order to solve this problem, it was proposed to use a pipe made of silver (Ag). The present inventor proposed an improvement in a process for producing a superconducting wire having an outer metal pipe made of silver in European patent application No. 88 401 064.6. In fact, if the silver pipe is used as the outer metal pipe, the oxygen contents inside the pipe can be controlled during the annealing stage since oxygen can pass through an wall of the silver pipe and can penetrate into the silver pipe. Namely, since the silver is an easily oxidizable metal, if a difference in oxygen density exists between inside and outside the silver pipe, oxidation propagate from outside to inside gradually, so that it looks like penetration of oxygen into the silver pipe.
However, the silver is not only a costly material which can not be used industrially but also can not prevent oxygen from re-escaping through the silver pipe from the resulting wire. Another problem of the resulting wire having such outer silver pipe resides in that the outer pipe is oxidized easily.
Therefore, an object of the present invention is to provide a novel process for producing an elongated superconductor which is composed of a sintered compound oxide or a metal oxide complex and which has an improved mechanical strength as well as a high critical temperature of superconductivity with a reduced cost.